Incorrect typing for Scope and Message in ASGIApp

[ods]

According to ASGI spec both scope and event message must be dictionaries, not an arbitrary mutable mapping:

• scope: The connection scope information, a dictionary that contains at least a type key specifying the protocol that is incoming
• receive: an awaitable callable that will yield a new event dictionary when one is available
• send: an awaitable callable taking a single event dictionary as a positional argument that will return once the send has been completed or the connection has been closed

and in other place:

scope must be a dict.

But in starlette they are declared as MutableMapping:

Scope = typing.MutableMapping[str, typing.Any]
Message = typing.MutableMapping[str, typing.Any]

Receive = typing.Callable[[], typing.Awaitable[Message]]
Send = typing.Callable[[Message], typing.Awaitable[None]]

ASGIApp = typing.Callable[[Scope, Receive, Send], typing.Awaitable[None]]

This actually breaks interoperability with other libraries that follows the spec. For example, the following code

from starlette.applications import Starlette
from httpx import ASGITransport

ASGITransport(Starlette())

gives the following error when type-checked with mypy:

error: Argument 1 to "ASGITransport" has incompatible type "Starlette"; expected "Callable[[Dict[str, Any], Callable[[], Awaitable[Dict[str, Any]]], Callable[[Dict[str, Any]], Coroutine[None, None, None]]], Coroutine[None, None, None]]"  [arg-type]
note: "Starlette.__call__" has type "Callable[[Arg(MutableMapping[str, Any], 'scope'), Arg(Callable[[], Awaitable[MutableMapping[str, Any]]], 'receive'), Arg(Callable[[MutableMapping[str, Any]], Awaitable[None]], 'send')], Coroutine[Any, Any, None]]"

The typing of ASGI app in httpx follows the spec:

_Message = typing.Dict[str, typing.Any]
_Receive = typing.Callable[[], typing.Awaitable[_Message]]
_Send = typing.Callable[
    [typing.Dict[str, typing.Any]], typing.Coroutine[None, None, None]
]
_ASGIApp = typing.Callable[
    [typing.Dict[str, typing.Any], _Receive, _Send], typing.Coroutine[None, None, None]
]

[sk-]

Note that these broader types also generated issues with the latest version of opentelemetry-python-contrib.

It would be great if the types could be fixed, or if the spec could be amended.

Maybe @Kludex could have some insights as he has driven some typing efforts, or @tomchristie who added said types.

[adriangb]

I don't think the issue is as simple as what you're proposing:
https://pyright-play.net/?strict=true&code=GYJw9gtgBALgngBwJYDsDmUkQWEMoCCKcANIQO4CGSMlARgDYCmZAwpQw-c2QLICutRk16UEydAChpAEybBYISigDOOPAAoxCAFxR2nbkwDaxmUgDGMYypggyROAF0nDqjSPGAcmBRMXAJRQALQAfFA%2BfjqSULFQAHSJ0pKUKnAoFlByChBwAPraGioWYAhMegJCzKLiqGg2dg7ETkFhEb7lMXGJ8dJ2ymq4MBq5BeIBkkA
https://mypy-play.net/?mypy=latest&python=3.12&gist=6e14a107050185893787deca37974b90

I see that you're using a version of MyPy that is 3 years old. Maybe try updating mypy or using pyright?

[ods]

Here is the results for the sample above, with mypy 1.10.0:

sample.py:4: error: Argument 1 to "ASGITransport" has incompatible type "Starlette"; expected "Callable[[dict[str, Any], Callable[[], Awaitable[dict[str, Any]]], Callable[[dict[str, Any]], Coroutine[None, None, None]]], Coroutine[None, None, None]]"  [arg-type]
sample.py:4: note: "Starlette.__call__" has type "Callable[[Arg(MutableMapping[str, Any], 'scope'), Arg(Callable[[], Awaitable[MutableMapping[str, Any]]], 'receive'), Arg(Callable[[MutableMapping[str, Any]], Awaitable[None]], 'send')], Coroutine[Any, Any, None]]"
Found 1 error in 1 file (checked 1 source file)

and pyright 1.1.366:

sample.py:4:15 - error: Argument of type "Starlette" cannot be assigned to parameter "app" of type "_ASGIApp" in function "__init__"
    Type "Starlette" is incompatible with type "_ASGIApp"
      Parameter 3: type "_Send" is incompatible with type "Send"
        Type "_Send" is incompatible with type "Send"
          Parameter 1: type "Message" is incompatible with type "Dict[str, Any]"
            "MutableMapping[str, Any]" is incompatible with "Dict[str, Any]" (reportArgumentType)
1 error, 0 warnings, 0 informations 

The error message from pyright suggests, that the problem is with Send, not Scope. Actually changing the signature of Starlette.__call__ to accept send: typing.Callable[[dict[str, typing.Any]], typing.Awaitable[None]] seems to fix typing errors with both mypy and pyright.

[Kludex]

What is the proposed solution here?

[ods]

This depends on our goal. If we want to be fully ASGI compliant, then we have to switch to dict in type declarations as suggested in pull request. But… having httpx doesn't follow the spec anymore too, just to be compatible with FastAPI… I already don't believe it has a chance to be fixed. Note, that the types are not more permissive here and in httpx. They're just different from ASGI, so both starlette/FastAPI and httpx may have problems with ASGI-compliant code.